It depends on the type of reading, the material that it is used – the subject chosen – the difficulty level of such texts, the level of complexity of the text itself, the richness of the text in terms of variability of the vocabulary.

A few months ago I read a New York Times article describing the number of different words used in different types of newspapers articles. It was stated that a very popular newspaper such as People is easy to read because each article uses the most common types of word (probably the most frequently used in the English language). Those articles are written for the general reader when other magazines underwent writing styles that are more deep in the overall extension of the vocabulary. Some article uses a wider spectrum of English words, the most of those words are used mainly in written contexts or formal environment like restrict professional fields – just think to the academic field, and the sub-areas of medicine, law or engineering for example.

To me the number of words a person can effectively learn depends, generally speaking, on the efforts the reader is willing to embrace. If the reader takes deep care about the text, he will stop any time he will find an new word. It is his own responsibility to jot down that word and try to familiarize with it with the support of a dictionary. On the other side, the reader can extrapolate the meaning of new word by inferring it from body of the text, the meaning of the paragraph or other elements such as the title of a specific chapter.

Answering to the question you can say that the rate at which learners learn new words depends on many factors: the more difficult the text the more the new words the reader can learn; the more the amount of curiosity, care, efforts of the learner, the more the words learned. The more the text is interesting to the reader the more the progress of the learner. The more the learner put a positive attitude on the text, the more the results potentially achieved.